1. Field of the Invention
The present invention relates to a method and a device for correcting an image captured by use of a fisheye lens or the like, and to a computer-readable non-transitory medium storing a program for correcting an image captured by use of a fisheye lens or the like. In addition, the present invention relates to a method and a device for generating a correction table for use in correction of an image captured by use of a fisheye lens or the like, and to a computer-readable non-transitory medium storing a program for generating a correction table for use in correction of an image captured by use of a fisheye lens or the like.
2. Description of the Related Art
In recent years, in some fields including the monitoring camera system and the video doorphone system, there have been demands for a wide field of view in one glance. Therefore, devices such as video cameras including a fisheye lens in an optical system and taking images with a horizontal angle of view of approximately 180 degrees have been brought to the marketplace. However, straight lines in the objective space (for example, straight lines in the real world) are generally greatly curved in the images taken by use of the above devices. Thus, geometrical correction for reducing the degree of distortion in the captured images is currently demanded.
Various projection methods including the equidistance projection, equi-solid angle projection, and orthographic projection are used in designing fisheye lenses. However, regardless of the projection method used in designing the fisheye lenses, it is possible to completely correct the distortion of the images of straight lines captured by use of a fisheye lens, by using perspective transformation based on design values and given parameters estimated by a conventional technique, as long as the angles of view to be represented in the corrected images are less than 180 degrees.
FIGS. 18A and 19A show examples of fisheye images. The example of 18A is an image of a hollow paper cylinder, and FIG. 19A is an image of an entrance hall. Further, FIGS. 18B and 19B respectively show examples of corrected images of the fisheye images of FIGS. 18A and 19A, where the fisheye images of FIGS. 18A and 19A are corrected by use of the perspective transformation such that the angle of view in the left/right direction is ensured to be wide. As is clear in the above examples, objects in the objective space which are to be represented by linear (straight) images are actually represented by linear (straight) images. However, in the corrected images, the peripheral regions are extremely enlarged and the depth perception is exaggerated. Consequently, the corrected images arouse a strong feeling of strangeness. This is a problem caused by a significant difference between the angle of view represented by the corrected image and the angle of view of viewing that image.
Japanese Patent Laid-open No. 2008-311890 (which is hereinafter referred to as JP2008-311890A) discloses a technique which is contrived by an attempt to solve the above problem. According to the technique disclosed in JP2008-311890A, a correction is made by using a cylindrical surface which is formed with a left portion, a front portion, and a right portion which are each an approximately flat surface and are smoothly connected, and the correction is made in such a manner that an angle of view in the left/right direction exceeding 180 degrees is achieved and each of linear objects which extend in the vertical direction or radial directions converging to the optical center (optical axis) are represented by linear images in the corrected image.